Insulated access doors for furnace walls are typically fabricated from concrete poured over a wire screen attached to the door structure and then cured. Alternatively, a plurality of laminated fire bricks are attached by complex lug arrangements to the door structure. Both of these arrangements are expensive and their installation is time-consuming, requiring the services of at least two mechanics over a period of days to complete the final installation.
In addition to the foregoing problems, these type of doors are difficult to repair and a boiler shut-down may be required when a door is removed for any length of time. Also, the doors presently in use are somewhat brittle and are damaged quite easily if not handled with extreme care, thus adding to the difficulties.
In an effort to overcome these defects, more recent insulated furnace doors utilize a plurality of cast ceramic materials. However, these materials have to be cast in place and then cured, which is time consuming. In addition, the use of multiple materials create inherent shear planes which lead to cracking and failure of the insulation.
Another problem associated with prior art doors of this type is that they have a relatively high thermoconductivity and thus often require the installation of an additional protective shield at the face of the door to accommodate excess temperatures encountered in the furnace. Of course, the installation and materials cost involved in such type of shields further adds to the expense involved.